Polyphase permanent magnet, brushless motors driven by a sinusoidal current offer the capability of providing low torque ripple, noise and vibration in comparison with those driven by a square wave current. Theoretically, if a motor controller can produce polyphase sinusoidal currents with the same frequency as that of the sinusoidal back EMFs, the torque output of the motor will be a constant, and zero torque ripple can be achieved. However, due to practical limitations of motor design and controller implementation, there are deviations from pure sinusoidal back EMF and current waveforms. The deviations will typically result in parasitic torque ripple components at various frequencies and magnitudes.
Another component of torque ripple in a conventional permanent magnet, brushless motor is cogging torque. Cogging torque is a result of the magnetic interaction between the permanent magnets of the rotor and the slotted structure of the armature. As the leading edge of a magnet approaches an individual stator tooth, a positive torque is produced by the magnetic attraction force exerted therebetween. However, as the magnet leading edge passes and the trailing edge approaches, a negative torque is produced. The instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots. The amplitude of the cogging torque is affected by certain design parameters such as slot opening/slot pitch ratio, magnet strength and air gap length.
One approach to reduce cogging torque is to skew either the stator teeth or the rotor magnet longitudinally, which provides for a gradual transition as the magnet moves under a stator tooth. However, due to practical factors such as magnetic leakage end effects, as well as skew variation in manufacturing tolerances, some undesirable cogging remains. An alternative approach is to employ a slotless armature, which allows for precise winding patterns in order to achieve a pure sinusoidal back EMF. In addition, the absence of slots in the armature eliminates the cogging torque resulting therefrom. However, the manufacturing process for slotless motors is not well defined and thus the manufacturing costs thereof may be prohibitive.
The problems and disadvantages of the prior art are overcome and alleviated by a brushless electric motor. In an exemplary embodiment of the invention, the motor includes a permanent magnet rotor having a plurality of magnetic poles thereupon and a stator. The stator further includes a plurality of stacked laminations defining a plurality of slots therein, and a plurality of fractional-pitch phase windings disposed within the plurality of slots. The motor has a non-integer slots-per-pole ratio, and a magnitude of 5th and 7th harmonic components of a fundamental frequency component of the motor are less than 0.3% and 0.1% of a magnitude of the fundamental frequency component, respectively. In a preferred embodiment, the stator is wound with three separate phase windings. Each of the three separate phase windings further includes three parallel branches, with each parallel branch having a plurality of serially connected coil segments.
In an alternative embodiment, each of the phase windings are wound in the stator according to the following pattern:
traveling inward, through a beginning slot, then;
traveling outward, through a second slot which is four slots counterclockwise from the beginning slot, then;
traveling inward, through the beginning slot, then;
traveling outward, through a third slot which is four slots clockwise from the beginning slot, then;
traveling inward, through a fourth slot which is one slot clockwise from the beginning slot; then
traveling outward, through a fifth slot which is five slots clockwise from the beginning slot.